From snake-oil salesmen and pool-hall hustlers to Nigerian princes and Spanish prisoners, scams were happening long before the Internet. Unfortunately, the Internet has only made things worse.

Modern scammers can reach billions of potential victims with a single message. And their scams are getting better every day. If you aren't paying attention, you could fall for a scam and not ever realize it.

One of the easiest places to encounter online scams is Facebook. Facebook encourages sharing, which means certain scams can travel far and wide.

These aren't harmless scams either. Some of them can install viruses that take over your account or steal your money. Yikes!

Here are four popular scams that should set off your warning bells as soon as you see them.

1. Free giveaways

The easiest scam to fall for on Facebook is a free giveaway. You'll see everything from gift cards to free tablets, laptops and smartphones. Who doesn't like free?

Just one catch. You have to give the "company" your information. Or you have to download a program to qualify.

This is a variation on a classic survey scam. These trick users into giving out their information or downloading malicious files onto their computers.

With the information you enter, a scammer has a foothold into stealing your identity. Entering your cellphone number often leads to bogus premium charges appearing on your wireless bill.

It's true that some companies do give away free stuff through Facebook. When they do, however, it's promoted on that company's official Facebook page.

If you check the company's page or website and don't see the giveaway, steer clear. Even if it's real, only enter through the company's official page. And only if it's a company you trust. Scammers like to set up fake sites and pages that mimic the real thing.

2. Viral videos

Almost as exciting as free gadgets is seeing the latest viral video. However, many supposedly salacious celebrity "videos" posted on Facebook aren't videos at all.

When you click to watch, you'll be asked to update your video player first. You'll even be provided with the updated program file. How helpful!

Of course, the program is really a virus. Plus, it will automatically share the scam with all of your friends.

This one is easy to avoid. Type the video's title into Google. You should see a link to it on YouTube. If the video isn't on YouTube or a legitimate news site, it's a scam.

3. Custom profiles

Another common scam offers to change your Facebook profile look or layout. A famous version was the Facebook Black scam. It supposedly gave you a slick, black color scheme.

These scams try to trick you into installing a rogue Facebook app. If you do, you give the scammer access to your personal data. It will also spam your friends to try and trick them.

Again, this scam is easy to avoid. Just remember that there's no official, or unofficial, way to change your Facebook layout. A program like Social Fixer can change the way you see it, but nothing will change the way other people see it.

4. Who viewed your profile?

Knowing who views your profile is the Holy Grail of social media. Who doesn't want to know who is cyber-stalking them?

That's why this scam has been around almost as long as Facebook. No matter how often Facebook says it isn't possible, people still want to believe it is.

So they gladly download apps or visit websites that turn out to be malicious. And they give full access of their Facebook profile to strangers. No good can come of that.

The best you can do is see who has unfriended you. Of course, that only works after you've activated one of the tracking services.

These are the most popular scams, but it's really just the tip of the iceberg. If you want to keep track of every scam happening on Facebook, the site Facecrooks is updated with scams as they break.